A common dielectric support for electronic components is often referred to as a circuit board. The traditional method of making these circuit boards is to impregnate woven glass fibers with epoxy resin precursors. These impregnated fibers are then pressed between two copper sheets at a high temperature for several hours to cure. The result is a copper clad board which can be further processed by etching, soldering and drilling to incorporate wires, electronic components or printed circuits thereon. The epoxy resin boards have good dielectric properties and high heat distortion temperatures. These traditional processes suffer in that the cure time is lengthy, which adds to the cost of the finished product and it is difficult to obtain shapes other than flat circuit boards by this process.
Another, more simplified method for producing circuit boards is by injection molding engineering plastics. Such methods are more rapid than traditional processes. The engineering plastics such as polyetherimide and polysulfone are distinguished from commodity plastics, such as polystyrene, in that they exhibit higher heat distortion temperatures, impact resistance and higher continuous use temperatures. For these features, a premium price is obtained. Because of this premium price, it is common practice in commercial industries to economize by balancing price versus the properties desired when selecting a polymer. Therefore, there is a continuing effort to provide materials with superior properties as well as a novel balance of properties for specialized applications.
For example, in preparing injection molded dielectric supports for electronic components, polymers with a high heat distortion temperature (Hdt) are required so that the integrity of the finished article is unaffected by the heat which generates from operation of the circuits. However, engineering polymers which exhibit high heat distortion temperatures characteristically exhibit correspondingly high glass transition temperatures (T.sub.g) and require more energy to process. Therefore, it may not be desirable to utilize materials with Hdt values higher than those necessary for the desired application. Polyetherimides are engineering polymers with excellent Hdt values for dielectric supports. However, they must be processed at high temperatures and high pressure because of their high Tg values and where the final product will not be exposed to extreme heat, the use of polyetherimides may be disadvantageous.
The present invention provides dielectric supports comprised of copolymers with a unique property profile. These copolymers have dielectric properties superior to epoxy resins, good Hdt valves and can be easily molded into finished products by utilizing a bulk polymerization process. The dielectric supports have the added features of low moisture absorption and good surface reactivity.
The copolymers used in the present invention are bulk polymerized to provide the desired dielectric supports. Common bulk polymerization techniques are reaction injection molding (RIM) and resin transfer molding (RTM). Both methods are known to provide shaped articles under relatively mild molding conditions, i.e., lower temperatures and pressures. In RIM processes, a low molecular weight polymer precursor or a reactive monomer is injected into a mold and is polymerized in bulk, i.e., without solvent or diluent to form the final product. The polymer precursor is typically a liquid monomer or an oligomer which exhibits a relatively low glass transition temperature and low melt viscosity, which simplifies transfer of the material into the mold. RTM is a method similar to RIM, the major differences being the speed with which reactants are transferred to the mold and the mixing of reactive components in that it is done at lower pressures. RTM is normally slower than the RIM process.
RIM and RTM processes are effective for a limited number of polymerized copolymers in that not all monomers/oligomers provide a solvent medium for a curing catalyst which generates polymer at a high degree of monomer conversion.
The copolymer used in the present invention is derived from two or more ring-opened polymerized norbornene-type monomers obtained by bulk polymerization. Minchak describes bulk polymerization and reaction injection molding of norbornene type monomers in U.S. Pat. No. 4,426,502. However, Minchak does not provide the dielectric supports of this invention which take advantage of the high cross-link density of copolymers derived from two or more norbornene type monomers to provide a unique property profile.